Butanediols, and in particular 1,4-butanediol (1,4-BG), find wide use in the chemical industry. 1,4-BG is used for a variety of purposes, and notable examples of its utility include use as a raw material for the production of a number of chemicals such as for the production of polyester. A number of processes are conventionally utilized to produce 1,4-butanediol. Conventional methods generally employ a hydrolysis reaction to produce 1,4-butanediol. For example, butadiene is reacted in an acetoxylation reaction with acetic acid (AcOH) and Oxygen then further hydrogenated to form 1,4-diacetoxybutane (1,4-DAB). 1,4-DAB is then further reacted with water (H2O) in liquid phase to produce 1,4-BG, 1,4-hydroxyacetoxybutane (1,4-HAB) and AcOH. Therefore, the product stream generally includes 1,4-HAB, AcOH, unreacted 1,4-DAB and various by-products. Purified 1,4-BG is typically recovered by multiple distillation steps. Typically, the 1,4-HAB and unreacted 1,4-DAB may be further reacted with H2O at different reaction conditions to form tetrahydrofuran (THF).
These conventional methods of producing 1,4-BG and additionally THF, are very energy intensive. Very large amounts of H2O are consumed in the hydrolysis reaction. One conventional method of reducing the amount of H2O used in the reaction is to employ more than one reaction/separation stage. An example of one illustrative embodiment of a prior art reaction system 10 is shown in FIG. 1a. Typically, multiple hydrolysis reactions are carried out in one or more reactors, and in FIG. 1a three reactor stages in series 12a-12c are shown, each having an associated separation stage 14a-14c. Large amounts of H2O, along with AcOH are separated in the separation stages and then conveyed to a waste water treatment plant (not shown) where the waste is treated which usually includes the recovery of AcOH in an acetic acid purification section (not shown). The boiling point of H2O and AcOH are lower than our desirable product(s), and thus they are typically removed as the distillate from the distillation tower. Although a very large relative volatility exists (i.e. the separation is relatively easy), it is necessary to boil off all of the H2O and AcOH. Hence, a large amount of energy is consumed.
To reduce the total amount of fresh H2O consumption, an alternative embodiment of the system 10 may be used; where typically, fresh H2O will only be added to the last stage of the reactors (i.e. 12c) in series, as shown in FIG. 1b. Then, the H2O together with AcOH formed in this separator of the last stage reactor, is recycled back to the previous reactor, or alternatively a recycled back to both of the previous reactors, and further, H2O and AcOH formed in the separator of the middle stage reactor may also be recycled to the first reactor as shown in FIG. 1b. This typical recycling system increases the total amount of H2O consumption slightly, however we can reduce the amount of total fresh H2O consumption significantly, consequently we will lower the energy consumption at the AcOH purification section and reduce the loading on the waste water treatment plant. However, regarding the total energy consumption for the overall system 10, it is still dominated by the total amount of H2O usage in the reactors, as a large amount of energy is needed to vaporized the H2O and AcOH from the mixture in the separators.
To understand the relationship of H2O usage (and therefore energy consumption) to the amount of 1,4-BG and 1,4-HAB production, lets start with the simple case of a system having one reactor stage with no recycle stream or system. FIG. 2a shows the performance of such a prior art reaction system. The x-axis shows the amount of H2O usage in the reactor, while the y-axis shows the amount of 1,4-BG production (curve A) and the corresponding production of 1,4-HAB (curve B) for a fixed feed amount of 1,4-DAB (in this example 12,800 kg/hr of 1,4-DAB as feed). It clearly shown that to achieve a typical desirable yield of 1,4-BG to 1,4-HAB (i.e., 1,4-BG/1,4-HAB mix) of a ratio of say 6:1 (5929 kg/hr-1,4-BG and 988 kg/hr-1,4-HAB), a large amount of H2O is used, in this case 145,000-kg/hr H2O. To determine the H2O efficiency of such as system, the amount of product, in this case 5929 kg/hr of 1,4-BG is divided by the total amount of H2O used (145,000 kg/hr) to arrive at a water efficiency of only 4.09%. This is illustrated in FIG. 2b where Curve I shows the production of 1,4-BG and Curve II is the water efficiency.
To reduce this amount of total H2O usage and/or energy consumption, one method used is to introduce an additional number of reactors to the system 10, as mentioned above. Moreover, since the system is employing multiple reactor stages, it is possible to reduce the total amount of fresh H2O by recycling the H2O from the separators to one or more of the previous reactors. However, in doing so, there is a very small penalty on the total amount of H2O usage.
There is another element that needs to be considered when determining the best overall performance of the system 10. This additional element is the capital cost of the system 10. For this hydrolysis reaction system, since it is an equilibrium reaction and the reaction conditions are close to equilibrium. Further, the amount of H2O flow in the system is almost equal to the total flowrate at the system due to the small value of equilibrium constant. Therefore, the amount of total H2O usage can be used as the measurement of the capital cost. This is because as the system uses more H2O, the reactor size is greater, and consequently the capital cost of the system is higher.
While the amount of H2O usage, and thus the energy consumption or costs, associated with producing 1,4-BG and additionally THF are reduced by employing more reactor stages (i.e. reactors/separators) and H2O recycle streams, the capital costs increase with the addition of these units. The prior art system configuration employing three reactors/separators and two H2O/AcOH recycle streams is desirable from both an energy and capital cost point of view. For the capital cost, although this prior art configuration uses two additional reactors as compared to the single reactor case, the total flow rate for the reactor system is significantly smaller than that for the single reactor case. Consequently, the size of the equipment is much smaller and this offsets the cost of the additional equipment necessary for the three-reactor configuration. Hence, in this instance the total capital cost for the three reactors with two H2O/AcOH recycle streams is lower than that for the one reactor with no H2O/AcOH recycle. However, there are many variables, constraints and tradeoffs between the energy costs and capital costs that must be considered.
Another technique that has been employed in the prior art is to recycle 1,4-HAB produced in the reaction back to the reactor. For example, Japanese Patent No. 55-16489 discloses recycling AcOH, diols and/or 1,4-HAB to a reactor. Japanese Patent No. 11-169435 describes recycling an effluent stream including 1,4-HAB to one or more reactors and focuses on reducing the amount of 1,4-HAB recycle. While these methods have provided an improved process, further improvement is desirable. Moreover, in these prior art patents it is believed that the desirable product is only 1,4-BG. Consequently, 1,4-HAB is considered as a waste and thus recycling it will be desirable. Accordingly, it would be highly desirable to provide a method and system for producing 1,4-BG, and optionally additionally THF, which promotes the more efficient usage of H2O and is capable of minimizing both the operating or energy costs of production and the capital expense of the system.
Accordingly, in summary, it is an object of the present invention to provide a method and system for producing 1,4-BG, and optionally additionally THF, in a hydrolysis reaction of 1,4-DAB.
It is another object of the present invention to provide a method and system for producing 1,4-BG, and optionally additionally THF, in a hydrolysis reaction where the operating or energy costs and/or capital costs associated with the system are reduced in comparison to the prior art systems.
It is another object of the present invention to provide a method and system for producing 1,4-BG, and optionally additionally THF, that promotes more efficient usage of H2O.
The inventors have discovered that the hydrolysis reaction may be shifted to favor the production of 1,4-BG with a significant reduction in the amount of H2O required to carry out the hydrolysis reactions. More specifically, the present invention provides a method and system for producing 1,4-BG in a hydrolysis reaction wherein 1,4-HAB is utilized as a starting material or reactant. The inventors have discovered that utilizing 1,4-HAB as a reactant in the hydrolysis reaction xe2x80x9cshiftsxe2x80x9d the equilibrium of the reactions to favor the formation of 1,4-BG. The equilibrium of the hydrolysis reaction may be shifted according to the present invention by providing 1,4-HAB to the reactor in a feed stream, or by recycling at least a portion of 1,4-HAB that is produced by the hydrolysis reaction back to the reactor, or by using a combination of both. The method and system of the present invention promotes a number of significant advantages. For example, utilizing 1,4-HAB as a reactant in the hydrolysis reaction to shift the equilibrium of the reaction significantly reduces the amount of H2O required to carry out the hydrolysis reaction. Furthermore, this significant reduction in the usage of H2O can be realized with system configuration comprised of less number of reactor/separators.
Accordingly, in one aspect, the present invention provides for a method of producing 1,4-BG in a hydrolysis reaction, comprising the steps of: supplying at least one feed stream including 1,4-DAB, 1,4-HAB and H2O to at least one reactor. 1,4-DAB, 1,4-HAB and H2O are reacted in the reactor to produce at least one effluent stream that includes 1,4-BG, 1,4-HAB, H2O, unreacted 1,4-DAB and AcOH. The effluent stream is supplied to a separation system having one or more separators where preferably at least a portion of the 1,4-HAB is removed from the effluent stream and recycled back to the reactor. Alternatively, 1,4-HAB may be supplied directly to the reactor as a feed stream, as opposed to being recycled from the process itself. In yet another embodiment, 1,4-HAB is supplied using a combination of recycling a portion and providing a portion in the feed stream. However, it is preferred to recycle at least a portion of the 1,4-HAB since it is a by-product of the reaction, and thus is readily available.
Of particular advantage, the system and method of the present invention is carried out such that the following equations are satisfied. Specifically, the inventors have developed upper (Max) and lower (Min) operating bounds which factor in the capital costs and energy use of the system, and then an operating condition (Var) is selected between such bounds as shown in the following equations:
Minxe2x89xa6Varxe2x89xa6Maxxe2x80x83xe2x80x83(1)
Preferably Min"" less than Var less than Maxxe2x80x83xe2x80x83(2)
where Max=(7.59Dxe2x88x920.76)/nxe2x80x83xe2x80x83(3)
Min=(3.79Dxe2x88x922.00)/nxe2x80x83xe2x80x83(4)
xe2x80x83Min""=(3.79Dxe2x88x921.46)/nxe2x80x83xe2x80x83(5)
and Var=B/Axe2x80x83xe2x80x83(6)
and D=174(C)/90(A)xe2x80x83xe2x80x83(7)
and where A is the feed rate of 1,4-DAB to the reaction system in kg/hr;
B is the feed rate of fresh 1,4-HAB to the reaction system plus the total amount of 1,4-HAB recycled to the reaction system in kg/hr;
C is the amount of 1,4-BG produced by the reaction system, and
n is the number of reactors within the reaction system.
In another aspect, the present invention provides a method of reducing the operating costs of a hydrolysis reaction to produce products including 1,4-BG wherein the cost of operating the hydrolysis reaction is defined in part by energy costs and capital costs. By using 1,4-HAB as a reactant, the equilibrium of the hydrolysis reaction is shifted toward increased production yield of 1,4-BG and decreased usage of H2O as compared to that in the absence of 1,4-HAB as a reactant. In this reaction, the energy costs are driven primarily by the H2O usage, and thus a reduction in the usage of H2O reduces the energy costs. Of further advantage, the reduction in the amount of H2O consumed in the hydrolysis reaction can provide a reduction in the capital costs of the system as the size of the reactors may be reduced.
In yet another aspect of the present invention, a hydrolysis system for producing products including 1,4-BG in a hydrolysis reaction is provided, comprising a reactor that receives reactants 1,4-DAB, H2O, 1,4-HAB and reacts said reactants to produce an effluent stream including 1,4-BG, 1,4-HAB, H2O, unreacted 1,4-DAB, and AcOH. A separation system receives the effluent stream and separates at least a portion of the 1,4-HAB from the effluent. Preferably a recycle stream is coupled to the separation system and conveys at least a portion of the 1,4-HAB back to the reactor as a reactant.